Windell Oskay of Evil Mad Scientist Laboratories removed the RFID circuit in a MasterCard PayPass and placed it into a Hello Kitty Pez dispenser. He calls it "Hello Credit."
One interesting thing is that the circuit board is incredibly thin -- approximately 1/4 the thickness of standard 1/16" circuit boards, which happens to be pretty close to the thickness of a standard credit card. It's flexible and fairly translucent. You do need to be a little bit careful because ceramic capacitors (like the ones on this board) have a tendency to break if you bend the board that they are soldered to.Link (Via Neatorama)
Of course, we can now give our RFID element any kind of a case mod that we see fit. One thing that would be easy to do is just to embed it in the side of a wallet. You could also embed it in any one of the millions of things that have become USB thumb drives in the last few years.